Griego, the 15-year-old son of an Albuquerque pastor, used two rifles to kill his mother, three siblings, and his father early Sunday morning, and loaded his van with the guns and ammunition he planned to use to shoot up a Walmart later that day, according to cops.

Griego and his girlfriend eventually went to Calvary Church in Albuquerque around 8 p.m. Sunday, nearly 18 hours after the spree began. There, Griego told church members his family had died in a car accident, prompting a church member to call police.

Police said Tuesday they were investigating whether the girl could face charges in connection with the crimes, but said today she would not.

"She has just been interviewed and she will not be facing any charges," Deputy Aaron Williamson, spokesman for the sheriff's office.

Investigators who went directly to Griego's house found the five family members dead at the home. Griego was found with a .22 rifle, an AR-15 rifle, two shotguns, and rifle ammunition in his van around the same time. After initially denying to police that he had anything to do with the murders, he admitted to killing the family members out of "frustration" with his mother.

He also told police he planned to shoot up a Walmart after killing his family, and that he also planned to kill his girlfriend's parents.

"Nehemiah said after killing five of his family members he reloaded the weapons so that he could drive to a populated area to murder more people," read a police report from the incident released Tuesday.

"Nehemiah stated he wanted to shoot people at random and eventually be killed while exchanging gunfire with law enforcement," the report said.

Relatives of Griego are described as stunned by his actions and "heartbroken over this senseless tragedy."

Though surviving relatives conceded in an unsigned "family statement" that Nehemiah Griego is a "troubled young man," the statement also described him as an outgoing boy who loved music and hoped one day to serve in the military.

"We know him as a bright, curious and incredibly talented young man. He was a brother, nephew, grandson and cousin," said the statement, obtained by the ABC News affiliate KOAT in Albuquerque from former New Mexico state Sen. Eric Griego, the suspect's uncle.

"We are deeply concerned about the portrayal in some media of Nehemiah as some kind of a monster," said the statement. "It is clear to those of us who know and love him that something went terribly wrong. Whether it was a mental breakdown or some deeper undiagnosed psychological issue, we can't be sure yet. What we do know is that none of us, even in our wildest nightmare, could have imagined that he could do something like this."

Griego is expected to face adult charges of murder and child abuse resulting in death. He waived his right to arraignment in adult court Tuesday and a judge ordered him held without bond.

The district attorney's office and Griego's public defender now are preparing to face a grand jury, KOAT reported.

"We never had a case like this, as far as I know, in the state of New Mexico," District Attorney Kari Brandenburg said, "so I can't compare this to any other case."

Griego has five older siblings who were not living at the home at the time of the shooting and were unharmed.

Eric Griego, the uncle who released the family statement, also released family photos late Tuesday, including one of his nephew in a tuxedo at a wedding and another of him playing a drum kit.

"From the time he was a young boy, his father Greg supported his love for music," the family statement said. "Thanks to his interest, practice and natural ability, Nehemiah has become a very accomplished musician. He plays guitar, drums and bass. For years he has played at youth and other church services at Calvary and elsewhere.